Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!dali.cs.montana.edu!uakari.primate.wisc.edu!zaphod.mps.ohio-state.edu!swrinde!elroy.jpl.nasa.gov!ncar!asuvax!mcdphx!hrc!gtephx!campbellb
From: campbellb@...!asuvax!gtephx (Brian Campbell)
Newsgroups: comp.sys.apple2
Subject: Re: ML subroutines (passing parameters in ML)
Message-ID: <1991May2.153346.1783@...!asuvax!gtephx>
Date: 2 May 91 15:33:46 GMT
References: <3397@kluge.fiu.edu> <13845@ucrmath.ucr.edu> <114343@tut.cis.ohio-state.edu>
Organization: gte
Lines: 18

In article <114343@tut.cis.ohio-state.edu>, meekins@anaconda.cis.ohio-state.edu (Tim Meekins) writes:
> In article <1991Apr29.203359.250@...!asuvax!gtephx> campbellb@...!asuvax!gtephx (Brian Campbell) writes:
> >BTW, if all of this is true, in a assembly routine, to by-pass the JSL
> >return address, would you typically use offsets of 3,5,7... to get word
> >parameters n,n-1,n-2,... where n is the number of data words pushed?
> 
> You've almost got it. The stack pointer points to where the next byte will
> go, NOT where the last byte is. To access the last byte pushed, use 1,s,
> NOT 0,s. Other than that, I believe you're on track. Also, if you map
> the DP to the stack (TCS,TCD), loading from Direct Page location $01 will
> access the last byte pushed.

Therefore, (1,s), (2,s), and (3,s) point to the RTL address, and offsets of 4,6,8,...
will point to parameters n,n-1,n-2,...  A handy way to access parameters, if they
are simple word values.  If they are pointers, then you would have to move them
into a direct page to use (zaddr) or [zaddr] addressing modes to get to the data.
The DP method can be more convenient (unless you also need to frequently access a DP
page as "registers").  BTW, don't you mean TSC?